This invention relates to distributed restoration algorithm (DRA) networks, and more particularly to a method and system therefor for determining the location of a fault when only a unidirectional alarm is received.
In a telecommunication network, certain portions of it may be provisioned with the ability to restore traffic that has been disrupted due to a fault or a malfunction at a given location of the network. Such portion(s) of the network that has been so provisioned with the restoration ability is known as a distributed restoration algorithm (DRA) network, or domain. Oftentimes, instead of being referred to as a domain of a telecommunications network, a DRA network can be considered to stand on its own as it is connected to other networks. Each node, or cross connect switch, in the DRA domain is equipped with an algorithm and associated hardware that allow it to begin to look for a new path for restoring the disrupted traffic when it senses a fault. Each of the nodes is interconnected, by means of at least one link or span, to at least one other node. Thus, the plurality of nodes of a network are interconnected to other nodes by a plurality of links. In addition to routing traffic, the links, via associated equipment, also provide to each node signals that inform the node of the operational status of the network. In other words, signals are provided to each node to inform the node that traffic is being routed among the nodes effectively, or that there has been a malfunction somewhere in the network and that an alternate route or routes are required to reroute the disrupted traffic.
Conventionally, when everything is operating correctly, absence of a special set of signals indicating a malfunction would inform those nodes that traffic is being routed correctly. However, if a fault occurs somewhere in the network that disrupts the flow of traffic, an alarm is sent out from the fault location and propagated to nodes of the network. Such alarm signal causes the equipment in the network downstream of the fault location to go into alarm. To suppress the alarm in the downstream equipment, a followup signal is sent.
As disclosed in the aforenoted copending application, the conversion of the sensed alarm signal by a pair of adjacent nodes that bracket the fault enables the management of the network to readily identify the location of the fault. This signal conversion technique works fine so long as bidirectional alarm signals are generated as a result of the fault. Putting it differently, as long as the pair of custodial nodes each detect an alarm signal resulting from the fault, the location of the fault can be isolated. But in the case where malfunction has occurred to the equipment, for example, that transmits the fault signal to the custodial nodes so that only a unidirectional alarm signal is sent to one of the custodial nodes, this signal conversion technique is at a loss for determining the location of the fault.
Therefore, there is a need for a method, and system therefor, of handling the failure of a malfunctioned link if the signal that indicates the malfunction is sent in only one direction.
An objective of the present invention is therefore to provide a method, and system therefor, of managing unidirectional failures so that such failures can be converted into bidirectional notifications of a fault to both custodial nodes that bracket, or sandwich, the location.
Upon receipt of an incoming alarm signal, each node of the present invention DRA network initiates a timer for a predetermined period of time, at the end of which, if the alarm signal is a unidirectional signal from the fault and assuming that the node that has received the alarm signal is one of the two custodial nodes that bracket the fault, it is assumed that the other custodial node should have likewise received the alarm signal. After the predetermined period of time, the custodial node that received the alarm signal sends a modified alarm signal, which may be in the form of an idle signal with an embedded message that indicates an alarm signal of a unidirectional failure, back along the path from where the alarm signal was received. If the failure is unidirectional, the transmission of the modified alarm signal effectively forces the failure to be bidirectional because it replaces the non-failed signal with the modified alarm signal. If the failure is bidirectional, the modified alarm signal is still sent, but it has no effect because it simply reaches the break/failure point and stops. Upon receipt of the modified alarm signal, the other custodial node, even if it did not receive the alarm signal directly from the fault, recognizes that a fault has occurred and would then, per the aforenoted copending application, convert the modified alarm signal into an idle signal to be propagated to nodes downstream thereof.
The present invention therefore converts a unidirectional failure into a bidirectional failure so that each node of the custodial pair of nodes that bracket the fault would receive an alarm indicating that a fault has occurred irrespective of whether or not the original alarm from the fault was a unidirectional alarm signal.